The invention relates to a method for inspecting an article for defects, wherein the article is scanned by means of a scanning light beam, the light scattered by the article is collected and the collected light is converted into an electrical scanning response signal which is fed to a processing unit for processing the scanning response signals, a defect in the article giving rise to a pulse in the scanning response signal.
An installation of this type is disclosed in European Patent Application EP-A 0 146 005.
In the case of the installation described in the abovementioned European Patent application, a light source in the form of a laser is used which directs a scanning light beam onto that face on one side of an article to be inspected facing said beam and focuses said beam on the surface thereof. A collection device in the form of an integrating sphere is also used. The scanning light beam enters the integrating sphere through an opening and leaves the sphere through an opposite opening in the sphere and is incident, as a focused beam, on the front face of the article. The light beam is directed at an angle onto the face of the article and the reflected light leaves the sphere via an opening which is located alongside the light beam inlet opening in the integrating sphere. The light which is scattered by the article, and in particular by a defect, is collected by the integrating sphere on a photoelectric converter, from the output of which an electrical scanning response signal can be obtained. If there is a defect, this causes a pulse scanning response signal to be generated. The scanning response signals are fed to a processing unit to determine whether or not there is a defect on the surface of the article.
All that is determined with this known installation is whether or not there is a defect on the surface of the article.
The aim of the invention is to provide a method of the type indicated in the preamble by means of which the location in the direction of the light beam, as well as in a plane perpendicular to the direction of the light beam of the defect in the article can also be determined.
Said aim is achieved according to the invention in that the light beam has a wavelength such that the article is transparent thereto, in that at least two scanning response signals are obtained, each of which being associated with a focal point located in the direction of the light beam at a different depth in the article, and in that the location of the defect in the direction of the light beam is determined from the steepness of the front and rear flanks of the pulses in the scanning response signals. By this means it is possible, for example, to determine whether a defect is on or in the article, so that a decision can be taken as to whether there is any point in cleaning.
In DE-A-39 26 349 a method and apparatus for inspecting articles for defects is described, in which it is known per sxc3xa9 to use 2 response pulses when the article is scanned. However, the depth of a defect in the article is determined from the distance between said response pulses or the amplitude of a response signal.
In a further development of the invention, the at least two scanning response signals are obtained by scanning the article at least twice, the scanning light beam having a different focal depth for each scan.
The use of scanning light beams of different wavelengths and with different focal depths also offers advantages for obtaining a number of scanning response signals. In this case one group of scanning light beams can be directed onto one side of the article and the other group of scanning light beams onto the other side of the article, in order to save space.
The location of a defect can be determined by interpolation of the focal depths between two test results for the steepness of the pulse scanning response signals.
In another embodiment the focal depth of the scanning light beam is gradually increased in the direction of said beam during repeated scanning of the article, the focal depth which is associated with the pulse scanning response signal having the steepest flanks being determined and used as an indication of the depth of the defect.
The invention also relates to an installation for inspecting an article for defects, comprising an article holder for holding an article to be inspected in an inspection plane, a light source, which emits a scanning light beam for scanning the article to be inspected, a collection device for collecting the light scattered by the article, a converter for converting the collected light into an electrical scanning response signal and a defect in the article giving rise to a pulse in the scanning response signal, and a processing unit connected to the output of the converter for processing the scanning response signal. This installation is characterised in that the scanning light beam has a wavelength such that the article is transparent to said beam, that means are present for obtaining at least two scanning response signals, each of which being associated with a focal point located in the direction of the scanning light beam at different depth in the article, and in that the processing unit is provided with a flank steepness measuring device for measuring the steepness of the front or rear flanks of the pulses in the scanning response signals fed thereto, and an evaluation device for determining the location of a defect from the measured flank steepnesses.
Illustrative embodiments of the invention which are preferably to be implemented are described in the further appended dependent claims.